IELTS Academic Reading Practice Test 15

A Cement is widely regarded as one of the most widely used materials in human history, consumed at a global rate of around four billion tonnes per year. Most concrete used in modern construction is made from Portland cement, a powder that hardens when mixed with water and aggregates such as sand and gravel. The chemistry behind this transformation is, in researchers' view, more elaborate than the simple appearance of the material suggests, and the environmental footprint of cement production has become a major focus of materials-science research. The dual concern with mechanical performance and environmental impact has produced an unusual convergence between engineering and climate research over the past two decades.

B Portland cement is manufactured by heating limestone with smaller quantities of clay, sand, and iron ore in a rotary kiln at temperatures approaching 1,450 degrees Celsius. At these temperatures, the raw materials are widely understood to undergo a series of chemical reactions that fuse the components into a hard, pelletised material called clinker. The clinker is then ground with a small amount of gypsum to produce the grey powder familiar to builders. The four principal minerals in modern clinker — tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite — together generally determine the cement's strength, setting time, and durability. The relative proportions of these minerals can be tuned by adjusting kiln temperatures and raw material chemistry to produce cements with different performance characteristics for specific construction applications.

C When water is added to cement, a hydration reaction begins almost immediately. The calcium silicate minerals react with water to produce calcium silicate hydrate, often abbreviated as C-S-H gel, which is widely said to be the binding phase that gives hardened cement most of its strength. The hydration reaction is exothermic — releasing heat that can crack large concrete pours if not managed — and continues for months, with strength rising gradually over the period. Civil engineers have, in many writers' view, developed extensive techniques to manage hydration heat in massive structures, including water cooling, partial cement substitution, and staged pouring.

D The environmental cost of cement production is widely regarded as substantial. Approximately one tonne of carbon dioxide is released per tonne of cement manufactured. About half this carbon dioxide is said to come from the chemical decomposition of limestone — calcium carbonate breaks down to calcium oxide plus carbon dioxide during the kiln process. The other half comes from the fossil fuels used to heat the kiln. Cement production is therefore widely understood to be responsible for roughly 7 to 8 percent of global anthropogenic carbon dioxide emissions. The scale of this footprint has, in many writers' view, elevated cement chemistry from a specialised engineering problem to a central concern of climate policy.

E Several approaches to reducing cement's carbon footprint are being actively explored. Supplementary cementitious materials including fly ash from coal-fired power stations, blast-furnace slag from steelmaking, and natural pozzolanas can substitute for a portion of the clinker in concrete with reduced or even improved performance. Calcined clay cements, developed by researchers at the École Polytechnique Fédérale de Lausanne, generally replace much of the clinker with clay calcined at lower temperatures than required for clinker itself. Carbon capture from cement plants, integration of supplementary materials with optimised mixes, and entirely new chemistries based on magnesium oxide or geopolymers are all reportedly at various stages of development.

F The fate of cement-based research is, in many analysts' view, bound up with the fate of urbanisation. Population growth and continuing urbanisation across the developing world generally mean that demand for cement and concrete is widely said to be likely to grow substantially over the coming decades, even as construction efficiency improves. Achieving net-zero emissions for the sector will probably require some combination of higher recycling rates, supplementary materials, alternative chemistries, and direct carbon capture, with no single technology likely sufficient on its own. The portfolio approach has been described by industry analysts as the most realistic pathway, in part because cement-sector emissions arise from both fuel use and chemical decomposition, neither of which can be addressed by a single intervention.

A A walk through almost any large city on a summer evening reveals a temperature several degrees higher than the surrounding countryside. This phenomenon, widely known as the urban heat island effect, was first systematically described by the British amateur meteorologist Luke Howard in his 1818 book The Climate of London. Howard is said to have documented that central London was on average 1.5 to 2 degrees Celsius warmer than rural areas immediately outside the city. Over the two centuries since, urbanisation has expanded enormously, and urban heat islands have, in researchers' view, become both more widespread and more intense. Howard's observations have been described by climate historians as a pioneering case of systematic urban meteorology, conducted decades before the discipline acquired a formal institutional home.

B The mechanisms that produce urban heat islands are widely regarded as well understood. Dark surfaces such as asphalt roads and tar roofs reportedly absorb a far larger fraction of incoming solar radiation than the vegetation and soil they replaced. Buildings generally reduce wind speed at street level, trapping heat. Concentrated waste heat from air conditioners, vehicles, and industrial processes adds further warming. Vegetation, which cools the surrounding air through evaporation from leaves, is widely understood to be greatly reduced in dense urban environments. Together these factors can produce temperature differences of up to 7 degrees Celsius between a city centre and its rural surroundings during heat waves.

C The impacts of urban heat islands on human health are, according to many epidemiologists, increasingly well documented. Elderly residents, children, and those with chronic illnesses are regarded as particularly vulnerable to heat stress, and the urban premium of a few degrees can be the difference between tolerable and dangerous conditions. The 2003 European heat wave, which killed an estimated 70,000 people across the continent, is widely said to have fallen most severely on city dwellers in poorly-ventilated apartments. Subsequent analyses identified the urban heat island as a substantial multiplier of mortality risk during such events. The 2003 episode has, in many writers' view, broadly become the canonical example used by public-health analysts to illustrate how urban form interacts with extreme weather to produce mass mortality.

D A range of interventions can, in researchers' view, reduce the intensity of urban heat islands. Cool roofs — light-coloured or specifically engineered to reflect more solar radiation — can substantially lower roof surface temperatures and reduce indoor cooling demand. New York City's "CoolRoofs" programme, launched in 2009, has coated more than 14 million square metres of roof area with reflective paint, contributing to measurable temperature reductions in nearby streets. Urban tree planting provides shade and evaporative cooling; cities including Melbourne, Singapore, and Medellín have launched large-scale tree-planting initiatives explicitly framed as climate-adaptation programmes.

E Beyond local interventions, planning and policy approaches are emerging. Green roofs — roof surfaces planted with vegetation — generally combine the cooling effect of vegetation with rainfall absorption that reduces stormwater run-off. Permeable paving and de-paving programmes typically replace impervious surfaces with greener alternatives. Some cities have updated building codes to require minimum cool-roof reflectivity or to limit dark-paved heat-storing surfaces. The cumulative effect of these measures can, according to many analysts, substantially moderate the urban heat island, although fully eliminating the effect is widely considered probably impossible given the inherent thermal properties of the built environment. The policy toolkit has, in many writers' view, matured to the point where municipal governments can choose among well-evidenced interventions according to local conditions.

F The relationship between urban heat islands and broader climate change is widely described as complex. Background global warming raises baseline temperatures everywhere, including cities. The urban heat island remains a roughly constant additional increment that can shift mild urban summers into dangerous ones as the baseline warms. At the same time, the policies that reduce urban heat islands also typically reduce greenhouse-gas emissions, so the two challenges are, in many analysts' view, addressed by overlapping interventions.

A Of all human behaviours, laughter is widely said to have perhaps the least obvious purpose. The patterned bursts of breath, the rhythmic muscle movements, and the distinctive sound generally carry no direct fitness benefit comparable to the obvious functions of eating, mating, or sleeping. Yet laughter is universal across human cultures, appears spontaneously in young children long before they understand humour, and is shared, in identifiable form, with several other primate species. Modern psychology and neuroscience have, in researchers' view, made considerable progress in identifying what laughter is, where it comes from, and what social functions it appears to serve. The behaviour has been described as one of the most informative natural experiments in non-verbal communication available for empirical study.

B The acoustic structure of laughter is, according to many phoneticians, remarkably consistent. A typical laugh consists of a series of short vocalisations, each roughly one-fifteenth of a second long, repeated at intervals of about one-fifth of a second. The fundamental frequency varies by individual and gender but follows characteristic patterns. The acoustic uniformity across cultures and even across primate species is widely said to suggest that laughter is a biological inheritance rather than a culturally learned behaviour, despite the cultural variation in what people find funny. The acoustic regularity has been cited as one of the strongest arguments that laughter is an evolved physiological response rather than a culturally shaped expressive convention.

C Laughter has been documented in non-human great apes for more than a century, but only in 2009 did a comprehensive comparative study by researchers at the University of Portsmouth establish the evolutionary relationship clearly. The team is said to have recorded laughter from infant chimpanzees, bonobos, gorillas, orangutans, and human babies during tickling and play. Acoustic analysis showed a clear progression from the more breathy, pant-like laughter of great apes to the more vocalised laughter of humans, with the timing of the change widely considered consistent with the divergence of these species over evolutionary time.

D Brain imaging has, in researchers' view, identified the neural substrates of laughter. Spontaneous laughter, the kind triggered by genuine amusement or social play, activates regions including the supplementary motor area, the anterior cingulate cortex, and the brainstem nuclei that coordinate breathing. Importantly, this differs from voluntary laughter, which is produced consciously without genuine amusement and is widely said to recruit a different motor pathway involving the primary motor cortex. Brain damage studies have documented patients who lose the capacity for spontaneous laughter while retaining voluntary laughter and vice versa, supporting the dual-route model. The double dissociation has, in many writers' view, broadly become a textbook example of how separable neural systems can produce superficially similar behaviours.

E The social functions of laughter have, according to many neuroscientists, attracted particular research attention. Laughter is widely regarded as highly contagious: hearing others laugh activates motor regions involved in producing one's own laughter, often before conscious decision. Brain regions involved in detecting social signals generally respond more strongly to laughter than to most other sounds. Functionally, laughter appears to communicate intent — a signal of shared enjoyment, of "this is play not threat" — and to strengthen social bonds. Robin Dunbar at Oxford has argued that laughter, like primate grooming, releases endorphins that contribute to group cohesion, but at far higher efficiency than physical grooming because many individuals can be groomed simultaneously by sharing in a joke.

F Laughter therapy and the medical literature on its possible health benefits have, in many clinicians' view, produced more contested findings. The popular claim that laughter substantially reduces stress hormones, boosts immunity, and improves cardiovascular health is supported by some small studies but has not been replicated in large rigorous trials. What is widely said to be well established is that laughter is a marker of social connection, and that social connection itself is regarded as strongly correlated with measurable health outcomes — possibly explaining the indirect health benefits of social laughter while leaving the direct physiological claims unproved. The pattern is common in clinical research on social behaviours: the indirect pathway through social connection is, in many writers' view, frequently more robust than direct claims about the behaviour itself.